In the packaging of certain types of foods, such as cookies, potato chips, and the like, it is common practice to employ a multilayer film having two or more polymeric layers wherein one of the layers is known to be an effective heat seal layer. In the packaging process, a supply of such a multilayer film can be shaped into a tube in a vertical form and fill machine. Marginal regions of the heat seal layer are brought into face to face relationship and heat sealed together. Thereafter, the packaging machine automatically forms a heat seal and makes a horizontal severance across the bottom of the bag. Next, product is dispensed into the open end of the tube and, thereafter, a second horizontal seal is effected across the tube with a simultaneous severing through the tube to result in a product packaged in a tube, heat sealed at both ends and along one seam at right angles to the end seals. While the food or other product is being dispensed into the package, air is also present in the package and this air assists in protecting and cushioning the product during subsequent shipment of the finished packages. During shipment of the product, particularly with larger size bags, e.g., those containing 6 ounces of product, the bags have a tendency to split or burst at the end seals. This effect is particularly noticeable where the product is shipped to high altitude locations or shipped through such regions, e.g., over mountain ranges.
A multilayer wrapping film of the type having the above-mentioned utility is described in U.S. Pat. No. 4,214,039, the subject matter of which is, in its entirety, incorporated by reference herein. U.S. Pat. No. 4,214,039 describes a packaging film structure comprising a polypropylene film substrate having a heat sealable vinylidene chloride polymer containing at least 50 percent by weight of vinylidene chloride. In between the polypropylene film and the vinylidene chloride polymer is a primer coat which consists of the reaction product of an acidified aminoethylated vinyl polymer and an epoxy resin. This primer coat enhances the bond strength between the polypropylene and the heat sealable vinylidene chloride polymer.
U.S. Pat. No. 4,345,004, incorporated herein by reference relates to a homopolymer polypropylene core layer co-extruded with an ethylene propylene copolymer which is biaxially oriented. The copolymer layer is corona treated and subjected to metal coating by vacuum deposition.
U.S. Pat. No. 4,439,493 discloses an oriented heat sealable structure which comprises a polyolefin film substrate, a layer consisting essentially of a random copolymer of ethylene and propylene having from about 0.5% to about 6% by weight of ethylene on at least one surface of the substrate, a primer coating on at least one surface of the random copolymeric layer and a heat sealable layer on the primer coating, wherein the heat sealable layer comprises an interpolymer comprising a minor amount of acrylic acid, methacrylic acid or mixtures thereof and a minor amount of neutral monomer esters comprising methyl acrylate, ethyl acrylate or methyl methacrylate. U.S. Pat. No. 4,439,493 is incorporated by reference in its entirety for all that it discloses.
U.S. Pat. No. 4,447,494, incorporated by reference in its entirety, discloses another oriented heat sealable structure, this one comprising a polyolefin film substrate, a layer consisting essentially of a random copolymer of ethylene and propylene having from about 0.5% to about 6% by weight of ethylene on at least one surface of the substrate, a primer coating on at least one surface of the random copolymeric layer and a heat sealable layer on the primer coating, wherein the heat sealable layer comprises a vinylidene chloride copolymer containing at least 50% by weight of vinylidene chloride.
U.S. Pat. No. 4,564,558 discloses a multilayer oriented heat sealable structure, comprising a polyolefin film substrate, a layer comprising a terpolymer of propylene with ethylene and butene-1, a primer coating on at least one surface of the terpolymer layer and a heat sealable layer on the primer coating, wherein the heat sealable layer is selected from the group consisting of a vinylidene chloride polymer layer and an acrylic polymer layer. U.S. Pat. No. 4,564,558 is incorporated by reference in its entirety for all that it discloses.
U.S. Pat. No. 5,093,194, incorporated herein by reference, discloses a multilayer oriented heat sealable structure, comprising a polyolefin film substrate, having on one surface a polymeric heat sealable layer comprising a terpolymer of propylene with ethylene and butene-1, and on the other a primer coating having thereon a water vapor and gas barrier layer comprising PVdC and inter polymer of acrylic acid and neutral monomer esters, e.g., methyl acrylate.
U.S. Pat. No. 5,194,318, incorporated herein by reference, discloses a metallized oriented film combination comprising a propylene homopolymer or copolymer substrate having a high density polyethylene skin layer with a thin metal layer deposited thereon. Optionally, the film combination can comprise a heat sealable polymer layer as well.
Despite these advances in the art, there exists a need for other multilayer heat sealable structures having improved properties, particularly with regard to film structures which can survive high altitude cross country overland shipping, without unacceptable levels of seal creep, preferably while maintaining a peelable seal. Moreover, there is a need for a film structure having enhanced puncture resistance, particularly for sharp-edged products, e.g., pretzels coated with salt crystals. Finally, it is desirable for such film structures to have high optic qualities, e.g., low haze levels.
It is an object of the present invention to provide an oriented polyolefin film which can be heat sealed.
It is yet another object of the present invention to provide a multilayer oriented film structure having adequate seal range and excellent bonding adhesion when laminated with polyethylene to a second film.
It is still a further object of the present invention to provide a multilayer oriented film structure for use in the packaging of foods having excellent sealability, seal integrity, peelable seal capability, transparency, and puncture resistance characteristics.
Other objects, aspects and the several advantages of the present invention will become apparent to those skilled in the art upon a reading of the specification and the claims appended thereto.